1. Field of the Invention
This invention relates to tinned steel sheet of high corrosion resistance having only a very small amount of iron dissolved through the edges, or any defective portion of its coating in a corrosive environment and a method of producing the same.
2. Description of the Prior Art
Tinplate has long been used for making various kinds of vessels because of its excellent appearance, corrosion resistance, workability, paintability and solderability. It, however, has a number of drawbacks, too. The biggest drawback is its very high cost of manufacture which is due to a sharp rise in the price of tin. A reduction in the amount of tin employed has been proposed to lower the cost of tinplate production, but has given rise to a lowering in the corrosion resistance of tinplate.
In order to solve this problem, there has been developed tinplate having a layer of nickel under a layer of tin, as disclosed, for example, in Japanese Laid-Open Patent Specifications Nos. 23091/1982 and 5884/1985. According to the teachings of these specifications, the combination of the undercoat and the tin layer and the formation of a uniform and dense alloy layer which is possible due to the presence of the undercoat enable a reduction in the amount of exposed iron or steel and thereby the production of tinplate of improved corrosion resistance. Tinplate of this nature is effectively used for making some kinds of vessels, but cannot be said to be fully satisfactory in corrosion resistance.
An attempt has also been made to use tinplate for making a vessel for alcoholic fuel. No satisfactory results, however, appear to have been obtained, since the pinholes which are formed in a coated layer on tinplate during its formation into a vessel gives rise to rusting or pitting corrosion. There is a strong demand for the tinplate which is highly resistant to pitting corrosion in any such defective portion of coating on the inner or outer surface of a fuel tank which may be used for holding alcoholic fuel, as well as gasoline.
Various types of cans have come to be manufactured for holding various kinds of substances and also for meeting the consumers' demand for products of higher quality. It is also necessary to use tinplate having a reduced thickness to lower the cost of vessel manufacture. In all of these cases, it is essential to improve the corrosion resistance of the vessels.
There is also an increasing demand for complexly shaped cans, such as cans of the necked-in type. They include heavily deformed portions of which a high degree of corrosion resistance is required, especially if they are used for holding a substance containing Cl.sup.- ions.
There are also known cans of the easy-to-open type having a tab which can be pulled to remove a lid portion for opening the can. In order to cope with a demand for cans of such type which are easier to open, it is usual to reduce the thickness of the lid, particularly its score portion. Improved corrosion resistance is required of these cans, especially in the score portion having exposed iron on the outer surface of the lid and the heavily deformed portion on the inner surface thereof. If the tab is formed from iron, it is necessary to ensure the corrosion resistance of its edge, particularly its rust resistance. Improved rust resistance is also required of a crown for a bottle.
Electric resistance welding (e.g., Soudronic welding) has recently come to be used for making cans. It is, however, essential to ensure the corrosion resistance of the welded portion.
Although the conventional tinplate is used for making various kinds of vessels as hereinabove described, it has the disadvantage that its tin layer is highly cathodic as compared with its base sheet, resulting in the dissolution of iron through its defective portions, such as pinholes, or scratches made during the manufacture of the vessel, which gives rise to rusting or perforative corrosion, depending on the contents of the vessel and its environment.